Power transformers and autotransformers and voltage regulators are used in an electric power generation, transmission and distribution system for power transfer, normally changing the output voltage value in relation to input. In order to allow controlling voltage supplied to load, its windings are equipped with taps which are connected to a CDC (Comutador de derivação em carga, hereinafter referred to by the English language equivalent, OLTC (On-Load Tap Changer)), which makes TAP change used with the energized transformer and with connected load, changing the coil ratio and consequently transformer output voltage.
In order to make the change and tap change the OLTC has a motorized drive mechanism and regulated by signals received in its inputs, to raise or lower the tap.
Currently in order to allow automatic voltage regulation supplied to load, transformers are equipped with a device called automatic voltage regulator relay, or voltage relay or simply relay, which send to the changer under load signals to lower or raise the tap position, so as to keep voltage in the load within programmed limits in relay parameterization.
In order to perform the automatic voltage regulation function, the voltage relays perform the transformer output voltage measuring through a power transformer (PT) and, the load current through a current transformer (CT). With these measurements, the voltage relay makes calculation of the voltage that actually reaches the load, by taking into consideration voltage drop in the transmission line, proportionate to the load current. Calculated voltage in load is then compared to a set of regulation parameters programmed in the voltage relay in order to determinate when there is need for change in OLTC tap. Said parameters include:
Rated voltage in load;
Tolerance margin, which is the allowed voltage percentage over and under rated voltage;
Timing for OLTC actuating in order to correct voltage, so as to avoid unnecessary tap change as a result of temporary voltage deviations, in which actuating time can be constant—regardless of the voltage deviation magnitude in relation to programmed tolerance margin (linear timing); or inversely proportional to voltage deviation magnitude in relation to tolerance margin (inverted timing); and
Parameters referring to voltage drop in line in a given load current condition (rated current), allowing the voltage relay to calculate actual voltage reaching the load.
Generally current voltage relays also have an input for making OLTC tap position measurement, through a position sensor existing in it. Measured tap position is then indicated to operator who may also perform OLTC command in manual manner, by blocking automatic voltage regulation commands. Based on this OLTC tap position measurements, some existing voltage relays also perform OLTC control in transformers electrically connected in parallel, therefore using the concept called Master-Command or Master-Follower. In this concept, one of the transformers in parallel is selected in master mode and the other in commanded mode. Every tap change performed by the master must be repeated by followers that lose their command independence to simply obey master transformer tap changes. In this way, all transformers will be in the same tap position, which prevents current circulation among windings in parallel, sending new trap change commands is blocked, preventing tap difference between transformers from increasing. 14) In operation in parallel of transformer banks consisting of individual single-phase transformers, additional care must be taken in order to ensure that the three single-phase transformers of which a bank consists are with their OLTCs in the same tap position, in order to prevent a voltage unbalance between the three-phase system phases. To this end a synchronism, check is performed between tap positions of the three single-phase transformers of the same bank, blocking OLTC operation and issuing an alarm is any tap discrepancy occurs between the bank phases. For maintenance and testing purposes only, when the bank is not electrically connected in parallel to others, is when OLTCs of each phase can be operated in different tap positions. In order to allow these tests and maintenances in existing system will have a selection, manually performed by the operator, where it is indicated whether the three transformers of the same bank must operate in the same tap position (“bank” mode) or in individual manner (“individual phase” mode).